Coolant bleeder valve purpose and operation

[joshmyzie2]

I'm leaking coolant out of my bleeder valve, so I've ordered a replacement. While I'm waiting for it to arrive, I've been trying to understand exactly what it's doing.


1. When the coolant warms, it expands and builds pressure. Is the tank at the same pressure as the rest of the system? Or is it at a lower pressure? Will coolant spray out if I open it while hot?

2. At 13PSI, the valve opens to release pressure. What's being released? Just water vapor? Are the additives vaporized as well? No liquid will be vented, right?

3. If water vapor is released every time the engine gets warm, why doesn't the system need regular refilling?

4. As the engine cools, the pressure will drop. At what point is air let back in through the bleeder? At atmospheric pressure? Negative pressure?

5. Why would I ever want to put the clip in the up position to force the bleeder open? Can't it bleed normally as the engine warms? I've read that people run with the clip up on the track; why?

Thanks!